Mica is classified as a phyllosilicate; its basic structural feature is a composite sheet in which a layer of octahedrally-coordinated cations is sandwiched between two identical layers of linked (Si,Al)O4 tetrahedra. The general formula of the mica structure can be found in Dana's New Mineralogy, R. V. Gaines et al., eds. (John Wiley & Sons, New York 1997), pages 1444-1446 and the structure can be written as:A0-5R2-3T4O10X2,where:
A=a large monovalent or bivalent ion (e.g. Na+, K+, Rb+, Cs+, Ca2+, Sr2+, Ba2+), or a partial vacancy (partial vacancy denoted by subscript “0”),
R=an octahedrally-coordinated cation (e.g. Li+, Mg2+, Fe2+, Mn2+, Zn2+, Al3+, Fe3+, Mn3+, V3+),
T=a tetrahedrally-coordinated cation (predominantly Si4+, with Al3+ and B3+), and
X=is an anion (predominantly OH− in minerals, but F− in glass-ceramics. X may also be partially O2−.)
Micas are extremely common in rocks, and numerous classification systems exist for them. In glass-ceramics, micas are typically classified as alkaline (containing alkali ions) and non-alkaline (containing no monovalent ions), and as trisilicic (where T4 in the formula above is (Si3Al)) and tetrasilicic (Si4). These compositional parameters can be varied to produce desired properties in a glass-ceramic.
Machinable mica glass-ceramics based on mica crystal phases were originally disclosed in the art more than thirty years ago [e.g. U.S. Pat. Nos. 3,689,296, 3,732,087, 3,839,055, and 3,756,838], and Table 1 below shows general formulas for trisilicic, tetrasilicic and non-alkali glass-ceramics having mica structures, the alkali-containing glass-ceramics being included in the trisilicic and tetrasilicic categories. These materials have found numerous uses based on their unusual capability of being machinable to high tolerances using conventional high speed metal-working tools. By suitably tailoring their compositions and nucleation and crystallization temperatures, a wide range of microstructures can be obtained, including the “house-of-cards” microstructure of relatively large mica crystals with high two-dimensional aspect ratios, which most enhances the inherent machinability of the materials (see W. Höland and G. Beall, Glass Ceramic Technology (Amer. Ceramic Soc., Westerville, Ohio, 2002), pages 7-9 and 236-241. Additionally, one can refer to U.S. Pat. No. 2,920,971 (Stookey), the basic patent in the field of glass-ceramics, which provides an extensive study of the practical aspects and theoretical considerations that must be understood in the manufacture of such articles as well as a discussion of the crystallization. More recent disclosures of machinable glass-ceramics (largely directed to glass-ceramics for dental applications) include PCT International Publication No. WO 2004/071979 A2 and U.S. Pat. Nos. 6,645,285, 6,375,729, 6,645,285, 4,652,312, 5,246,889 and 4,431,420. The foregoing machinable mica glass-ceramics typically have an inherent white color. For certain applications, however, especially for consumer-oriented products or dental applications (hued to match existing teeth), a colored machinable glass-ceramic is desired.
While existing machinable glass-ceramic have many valuable properties, improvement are still highly desirable, particularly with regard to strength and fracture toughness. The present invention is directed to a machinable glass-ceramming having improved characteristics, including high strength and high fracture toughness.
TABLE 1General Types of mica structures in glass-ceramics (prior art)TrisilicicTetrasilicicNon-alkaliWt %mica GCsGCmica GCSiO225-60 45-7530-65 Al2O35-250-55-26B2O35-150MgO5-25 8-3010-35 F4-20 3-154-12K2OSee below 2-20Li2O0-7 0-5R2O2-20See belowZrO20-7SrO5-25BaO0-25WhereThe sum of Al2O3 + B2O3 = 15-35%Na2O = 0-5The sum of MgO + Li2O = 6-25%R2O = alkali oxides:K2O = 0-15%Na2O = 0-15%Rb2O = 0-15%Cs2O = 0-20%